1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a single piece turbine rotor blade having impingement cooling for the airfoil and convection cooling for the platform.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
Turbine blades are cooled using a combination of convection cooling, impingement cooling and film cooling in order to control the blade metal temperature in order to prevent thermal damage such as erosion and to provide for a long blade life. The internal cooling air passages and features are formed using an investment casting process in which a ceramic core is used to form the internal cooling air passages within a metal blade. The ceramic core is placed within a mold and liquid metal is poured over the ceramic core and solidified to form the blade. The ceramic core is then leached away from the solidified blade to leave the internal cooling air passages within the blade. Film cooling holes are then drilled into the blade using a laser or an EDM probe.
Use of the ceramic core in the investment casting process to form a blade has two major limitations. One is that the internal cooling air passages and features cannot be formed in complex shapes because of the way that the ceramic core is formed. The ceramic core is cast within a mold such that the features must be aligned with a pulling direction of the mold pieces. For example, ribs that extend from a pressure side wall to a suction side wall must be parallel to the pulling direction of the mold. Also, the ribs must not be angled or tapered that would prevent the mold piece from being pulled away from the hardened ceramic core. Second, features smaller than around 1.3 mm in diameter cannot be cast successfully because of the weakness of the ceramic material in the ceramic core. The relatively heavy liquid metal that flows around the ceramic core features would break off such small ceramic core pieces such as the pieces that form impingement cooling air holes. Broken pieces within the ceramic core due to the liquid metal flowing would result in defective cast blades.